The composition of this invention is referred to as a “dispersion” and not a “suspension” because solid particles, typically crystals of DBNPA (which is not a salt), in this dispersed phase, are substantially homogeneously distributed in a continuous phase of brine to yield a stable dispersion, when held as such, in an air-tight closed container without shaking, for a relatively short period, typically no more than three (3) months. By “substantially homogeneously distributed” is meant that there is less than a 10% difference in concentration of DBNPA particles in a layer in the uppermost 10% of the height of the dispersion in a 500 ml graduated cylinder, and a layer in the lowest 10% of the graduate stored in a closed container at 25° C. In a suspension, particularly one in the form of a gel, the DBNPA particles will remain homogeneously dispersed, if not indefinitely, for a very much longer period than they will in a dispersion.
DBNPA, also referred to as 2-cyano-2,2-dibromoacetamide, is a particular haloacetamide widely used as a biocide to control the accumulation of bacterial slime in aqueous systems. Slimes are generated due to the growth of bacteria and/or fungi in aqueous systems including process water used in paper mills, cooling water towers, and various equipment used in unit operations; and, in aqueous systems for the production of cutting oils, textile oils, and water-based paints, inter alia. DBNPA in a low concentration in the range from 10 ppm (parts per million) to 1000 ppm in process water, is found to eliminate putrefaction and contamination which occurs due to the growth of bacteria and fungi. Though rapid degradation of the DBNPA in an aqueous stream, in the range of from 1-hr-12 hr, is a desirable environmental attribute after the DBNPA has functioned as a biocide, such desirable degradation also results in accelerated degradation during storage of the dispersion prior to its use.
To deliver a precise dosage amount of the DBNPA into such aqueous systems, a user would like to dilute a pourable and pumpable aqueous liquid concentrate without any problems associated with dispensing the biocidal concentrate; and, which concentrate when stored generates no undesirable byproducts; and when used has no undesirable effect on the environment. Lastly, a user would like to purchase the desired DBNPA active ingredient without paying for anything which the user does not, or cannot use, or, from which the user cannot benefit in some way.
At present, DBNPA is sold as a stable liquid in a non-aqueous solvent such as a polyalkylene glycol to avoid the hydrolysis problem with free water. Instead of the glycol providing the user with a benefit, the glycol provides nutrition for the bacteria the DBNPA is being used to kill and introduces a small but highly undesirable amount of volatiles. For example, Pluriol® E 200 LS and Pluracol® E 200 LS polyethylene glycol (PEG) from BASF Corp. introduces <1% volatiles. To cut the costs incident to using DBNPA in glycol solvent, Segall et al in U.S. Pat. No. 5,070,105 disclosed one could add some water provided one used various stabilizers. Though degradation is decreased, the cost of glycol, coupled with the cost of removing glycol (not ingested by the microbes) from water discharged into the environment, is still high. This high cost stemming from the high chemical oxygen demand (COD) of the glycol present, makes such non-aqueous, organic solvent formulations undesirable.
Like the solution in glycol, any stable aqueous dispersion is also to be diluted with water to make the biocidal solution. By “stable” is meant both physically and chemically stable. By “physically stable” is meant that less than 3%, typically from about 0.01% but less than 3%, of the solid particles of the DBNPA settle out after being stored in a closed container at a temperature in the range from about −5° C. (minus 5° C.) to 35° C. for a specified period, up to 3 months, during which the dispersion will be used. By “chemically stable” is meant that from about 0.01% to less than 3 wt % of DBNPA is lost due to hydrolysis during the same specified period, up to 3 months, at the same temperature. An “environmentally friendly” viscosifier refers to one which has minimal adverse impact on the natural environment such as is obtained with naturally occurring materials, or man-made materials which behave like naturally occurring materials.
Either a mass of DBNPA solid particles, or a compact or tablet made with the solid particles, is difficult to use, therefore undesirable, as is an aqueous gel requiring fluidization before it can be pumped.
In his U.S. Pat. No. 5,627,135 (“'135 patent” for brevity), Gartner stated that “It would be desirable to discover liquid formulations of DBNPA that utilize water as a suspending medium and in which the DBNPA is protected to prevent or reduce the decomposition or degradation thereof” (see '135, col 2, lines 12-15). He recognized that this type of formulation would (i) not only reduce the COD as compared to the present commercial formulations which employ polyalkylene glycols, but such a formulation would also be less expensive; (ii) that it would be advantageous if a wide range of concentrations of DBNPA could be employed in the formulations; and, (iii) that it would be desirable if the “formulations were insensitive to changes in temperature and electrolyte concentration” (see '135, col 2, lines 22-23). Note that Gartner referring to an electrolyte, states that “The formulations are substantially insensitive to changes in temperature from about 0° C. to about 100° C. and to changes in the electrolyte concentration.” (see '135, col 2, lines 36-38), but does not state what the electrolyte is, what its function is, or how much of it is used to do whatever it is supposed to do.
Gartner's solution to the problem was an antimicrobial formulation in water, which formulation comprises from at least about 3 to 70 weight percent (“wt %”) DBNPA suspended in at least about 30 to 97 wt % water in the presence of a suspending amount of a thixotrope that exhibits Ellis-Plastic behavior at a pH of from about 1 to about 4. In addition to DBNPA being suspended in a thixotrope, he specifically requires that the thixotrope exhibit the specified Ellis-Plastic behavior. Thus his “liquid formulation” was not a “liquid” until the gel was subjected to enough stress to fluidize it into a liquid state.
That Gartner relies on forming a gel, which exhibits Ellis-Plastic behavior at a pH in the range of from 1 to 4, to negate the hydrolysis is evidenced by his statement that:
“These thioxotropes typically exhibit a yield value which exceeds the force of gravity acting on the DBNPA particles thereby allowing the DBNPA particles to be suspended in water and thus protected from the degrading effects water usually has upon DBNPA.” (see '135, col 3, lines 55-60).
Thereafter, with respect to “A series of mixtures ranging from 0.1 to 2 weight percent thixotrope in water”, (see '135, col 4, lines 3-5) he states: “Typically, a few minutes of agitation is sufficient to achieve a uniform suspended mixture in the form of a gel for each formulation. However, three hours should pass before Step 2 is undertaken in order that the formulation reaches an equilibrium at which it will exhibit its final thixotropic properties.
2. After three hours have passed, the formulations can now be tested for suitability hi the invention. Gentle agitation is applied to the formulations. If little or no flow occurs upon agitation of the formulation then the thixotrope is not suitable for use in the invention. Suitable thixotropes (assuming they exhibit suitable yield value and stability as determined in step three below) should cause the formulation to liquefy and flow upon agitation and return to its gel form almost immediately upon cessation of agitation.” (see '135, col 4, lines 8-22).
With respect to such a thixotrope, he defines “thixotrope exhibiting Ellis-Plastic behavior” as referring “to compounds or mixtures of compounds which cause a formulation to exhibit the following properties. First, the formulation must form a gel which liquefies when agitated, yet returns to the gel state when it is at rest. Second, in contrast to most liquids which will flow when subjected to any shear stress, i.e., force applied to the liquid, no matter how small the stress, formulations of this invention require some minimum amount of shear stress in order to liquefy the formulation and cause it to flow. This minimum amount of shear stress is called the “yield value” and it varies as the particular thixotrope and its concentration vary. The yield value of the thixotrope must be high enough to suspend DBNPA particles in water. This means the yield value must exceed the force of gravity acting on the DBNPA particles or the DBNPA will settle to the bottom.” (see '135, col 2, lines 58 to col 3, line 6).
Note that in his Table II, Gartner shows he made two aqueous suspensions, Examples I and II, each having about 50% DBNPA; 0.75% xanthan gum; 0.25% locust bean gum; and 49% water. One (Example II) of the suspensions was aged for 15 months before both were tested for antimicrobial activity; the other was not aged. There is no indication as to whether (I) and (II) exhibited Ellis-Plastic behavior at any pH.
Tables IV and V show that (I) and (II) are comparably effective. But there is no indication as to how much of the DBNPA in (II) settled, nor how much had been lost to hydrolysis.
Gartner states “A series of mixtures ranging from 0.1 weight percent to 2 weight percent thixotrope in water are thus prepared. To each of these mixtures, a predetermined amount of solid DBNPA is then added with agitation to prepare a series of formulations containing from 3 to 70 weight Percent DBNPA. Typically, a few minutes of agitation is sufficient to achieve a uniform suspended mixture in the form of a gel for each formulation. However, three hours should pass before Step 2 is undertaken in order that the formulation reaches an equilibrium at which it will exhibit its final thixotropic properties.”
Surprisingly, the same or equivalent gum(s), namely, xanthan and locust bean gums which met the requirements (in the '135 patent) to function as a thixotrope, alone or in combination, which exhibits Ellis-Plastic behavior in the '135 patent, when used in an amount totaling less than 1.0 wt % in a dispersion specified herein, fails to form a thixotrope which exhibits Ellis-Plastic behavior. Nevertheless, Gartner states “Generally, the suspending amount is at least about 0.03 weight percent of the total suspension, preferably at least about 0.8, to at most about 4, preferably to at most about 2 weight percent of the total suspension.” (see '135, col 4, lines 57-63).
Assuming, if for no logical reason, one considered using a liquid brine in lieu of water, one would expect that, the solubility of DBNPA in brine being even lower than that in water, the tendency of solid DBNPA particles, which have a true density of about 2.35 g/cc, to settle out of the brine dispersion would be much greater than the tendency to settle out of water or brine. In addition, since the pH of the brine before addition of the DBNPA, would be above 4, typically in the range from above 4 but no higher than 6, one would expect that the novel aqueous dispersion in brine would be susceptible to hydrolysis (as stated by Gartner and Carlson).
To avoid the hydrolysis problem in DBNPA, Iwasaki et al in U.S. Pat. No. 4,770,694 teach a biocidal suspension consisting essentially of (a) from 10 to 60 wt % of DBNPA, and (b) from 0.2 to 20 wt % of a phosphate ester salt of a non-ionic surfactant solid as the essential ingredient, thickened with a polysaccharide or gum, e.g. 0.5 part of xanthan gum or guar gum, the latter being well-known thickening agents in aqueous systems. They indicate that their aqueous suspension solves all problems when a specific surfactant is used as the dispersant, and has long shelf life and suspension stability upon dilution (see '694, col 1, lines 52-57). In '694, col 4, lines 25-38, they generically set forth the ingredients to make an aqueous biocide suspension composition having good shelf life and excellent flowability—but they do not state what “good shelf life” means, nor how much the biocide settles. The statement relating to “suspension stability upon dilution” is to be weighed in light of data in U.S. Pat. No. 4,800,082 Karbowski et al, and their statement that “The half-life of DBNPA in this tower was determined to be very short, estimated at less than one hour.” (see '082, col 10, lines 32-33). The “tower” referred to is a Marley cooling water tower.
Reverting to the '082 patent to Karbowski et al, they teach making a solid compact of plasticized water-insoluble cellulose ether particles, e.g. methylcellulose, into which DBNPA is reversibly diffused, the compact to be used without a pump, an eductor or a similar dispersing apparatus (see '082, col 2, lines 28-31). Though they disclose a compact of (a) about 1 to 90 percent by weight of a halogenated amide antimicrobial compound, and (b) about 10 to 80 percent by weight of a suitable hydrophilic polymer selected from the group consisting of a natural water soluble cellulosic polymer, a synthetic water soluble cellulosic polymer, gelatin, maltodextrin, xanthan gum, carrageenan, carboxymethyl guar, hydroxypropyl guar and carboxymethyl galactomannose, they do not suggest that the compact is dispersible in water. Nor do they suggest that they could essentially negate degradation by hydrolysis if they used a viscosified brine in which to form a stable dispersion with less than 1 wt % of a hydrophilic polymer such as a gum.
Instead, Karbowski et al suggest placing their solid composition in a perforated polyethylene container through which water is flowed, and water gets treated as it flows over a tablet. They limit the flow of water over the surface of their composition which allows for an even longer treating time period (i.e., prolonged sustained release).
Though they recognized the problem of degradation of DBNPA (see '082, col 1, lines 19-27) they never suggested they could disperse the solid particles of DBNPA in a viscosified brine and provide either a chemically or physically stable dispersion. Repeating one of their examples provides evidence that their dispersion is neither homogeneously dispersible nor sufficiently physically or chemically stable, as required and defined above.